1. Field of the Invention
The present invention relates to wireless communication, and, more particularly, to a method and apparatus for performing a hybrid automatic repeat request (HARQ) in a wireless communication system based on Time Division Duplex (TDD).
2. Discussion of the Related Art
Long Term Evolution (LTE) based on 3rd Generation Partnership Project (3GPP) Technical Specification (TS) Release 8 is the leading next-generation mobile communication standard.
As disclosed in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, in LTE, a physical channel can be divided into a Physical Downlink Shared Channel (PDSCH) and a Physical Downlink Control Channel (PDCCH), that is, downlink channels, and a Physical Uplink Shared Channel (PUSCH) and a Physical Uplink Control Channel (PUCCH), that is, uplink channels.
A PUCCH is an uplink control channel used to send uplink control information, such as a Hybrid Automatic Repeat reQuest (HARQ), an acknowledgement/not-acknowledgement (ACK/NACK) signal, a Channel Quality Indicator (CQI), and a Scheduling Request (SR).
Meanwhile, 3GPP LTE-Advanced (A) that is the evolution of 3GPP LTE is in progress. Technology introduced into 3GPP LTE-A includes a carrier aggregation.
A carrier aggregation uses a plurality of component carriers. A component carrier is defined by the center frequency and a bandwidth. One downlink component carrier or a pair of an uplink component carrier and a downlink component carrier correspond to one cell. It can be said that a terminal being served using a plurality of downlink component carriers is being served from a plurality of serving cells.
In a Time Division Duplex (TDD) system, the same frequency is used in uplink and downlink. Accordingly, one or more DL subframes are associated with an UL subframe. The “association” means that transmission/reception in the DL subframe is associated with transmission/reception in the UL subframe. For example, when a transport block is received in a plurality of DL subframes, a terminal sends HARQ ACK/NACK (hereinafter referred to as ACK/NACK) for the transport block in an UL subframe associated with a plurality of DL subframes. Here, a minimum time is necessary to send the ACK/NACK. This is because the time taken to process the transport block and the time taken to process the ACK/NACK are necessary.
Meanwhile, a plurality of serving cells can be introduced into a TDD system. That is, a plurality of serving cells can be assigned to a terminal. In this case, in the prior art, it was assumed that the same uplink-downlink (UL-DL) configuration was used in all the serving cells. The UL-DL configuration is information indicating whether each subframe within a radio frame used in TDD is an UL subframe or a DL subframe. In the next-generation wireless communication system, however, to use different UL-DL configurations in serving cells is also taken into consideration. In this case, how an HARQ will be performed using what method is problematic.